This post is full of complete nonsense.
"Still unable to gain smp support or clock speeds with the 750 family for the g5" <- what does that mean?
lots of mixing up between g3 and g4 and g5, or the idea that a g3 outperforms a dual g4 which is just damn ridiculous.
G3 is a powerpc 750, G4 a new design (power pc 7400) and G5, well a radically different thing, it's derived from the POWER4 and was powerful (had a dual core version too) but was too hungry for a laptop (no mac mini with it either)
Xenon and PPE were a new design that didn't go anywhere - though there's POWER6 sharing a basic philosophy of in-order and high clock, with the difference that it was made to have real peformance.
Amazingly, after the incoherent and undecipherable stuff you've written, what you say here makes mostly sense and I mostly agree with it. Those are reasons to think it's not a 750, it may be a custom design (IBM can afford this) but it may re-use whatever concepts for earlier CPUs or the ppc 47x which was being developed, I don't know.
The radiation hardened version dates back to 2001 so it's not much secret, given it's referenced on wikipedia with details there and a list of notable space probes using it. 110 to 200MHz by the way and 250 or 150nm process.
http://en.wikipedia.org/wiki/RAD750